The field of the present invention is refractory mixes and more specifically relates to refractory ramming mixes which are useful for the preparation of monolithic linings for furnaces, kilns, hot metal ladles and other refractory applications where resistance to high temperature is required. The present invention is not limited to ramming mixes, however, and may also be applied to the fabrication of refractory bricks and refractories of other structural shapes.
Mixes containing a high percentage of alumina are formed into a refractory composition by the addition of a binder such as phosphoric acid followed by drying and firing steps. Other ingredients such as clay are often added to improve the plasticity of the mix and relatively pure alumina and natural minerals containing a high percentage of alumina generally make up a large part of the refractory composition.
In order to fabricate a monolithic or other refractory surface of the type discussed herein, the dry ingredients (e.g., the high alumina mix) are mixed and the binder (typically a liquid having a relatively high viscosity) is mixed with th dry ingredients. A plastic or moldable mix results which is usually wrapped in a plastic sheet to prevent drying. Alternatively the plastic mix may be used shortly after mixing to permit forming the desired shape. This plastic mix is next allowed to dry at a relatively low temperature and then fired at a temperature usually in excess of 2,000.degree. F. to form the final refractory. During this firing step the binder and alumina react to form a strong refractory.
In the past, refractory mixes such as those described above have had several shortcomings. The use of phosphoric acid as a binder has often led to cracking upon drying. Furthermore, there has been a need for higher strength after firing. Attempts to modify the phosphoric acid to form an improved binder has led to high processing costs. Furthermore, such binder modifications have often led to undesirable air set properties. Further improvement in the resistance of the fired refractory to alkali attack is also needed for many end uses. The shelf stability of many prior art mixes has been limited resulting in a loss of bonding strength after prolonged storage of the plastic ramming mix. This is also manifested in a hardening or setting of the mix prior to use, making it impossible to form the mix into a desired shape. Lastly, there is a need to decrease the cost of the binder composition over both the modified phosphoric acid type binder and phosphoric acid itself.